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Full text of "Handbook Of Chemical Engineering - I"

CONCENTRATION                                     325
reducing agencies as frequently as possible. In preliminary crushers the crushing faces or masses are of no greater weight than is necessary to provide for wear and to resist the crushing forces. In theoretical power discussions all the moving parts of the reducing machines preliminary and secondary must be considered the crushing faces or masses. Bearing this in mind it may be stated in a general way that the weight of the crushing masses to the ton of ore crushed are inversely proportional to the size of the particle through the whole range of crushing and grinding machines. The crushing masses are least in weight in crushers and greatest in tube mills. Offsetting the inefficient application of power what this statement implies is that greater power is consumed in bearing friction in the cases of the preliminary machines. It must be evident that if there is reduction by direct application of pressure this pressure must be carried back to the bearings and result in great power consumption in the way of friction. In the case of stamps and ball and tube mills charged with their grinding balls or pebbles there is practically no greater power consumption between running empty and full. In these cases the power losses due to friction are caused directly by the weight of the moving or power masses. The power for useful work, that actually used for reduction, is a part of the power required to raise the stamp or balls or pebbles to the point where they fall on the material to be reduced.
The chief inferiority of the principles employed in secondary crushing lies in the fact that they lead to much power lost in uselessly moving about the material to be crushed. In a test made with Cripple Creek ores with a large Chilean mill the power required for driving the mill empty was 9 hp. With a light feed the power increased to 28 hp. With heavy feeding to 63 hp.; and at the limit of loading the power required was 70. In the case of this grinding machine the bulk of the power used is employed in raising the wheels, and the height to which they must be raised increases with the load. At the same time as must be evident, great increases in power result from moving the greater masses of ore in the mill as the load increases.
Some authorities favor carrying roll crushing down to H 5 of an in. but the limit for these machines is today owing to other practical considerations about 34 in. At one time there was and there is still an active school which advocates taking material directly from large crushers and feeding it to ball mills for reduction to sand sizes. This practice led to some disastrous deficiencies in expected tonnages. The tendency today is to consider material 1 in. in size and smaller as the maximum in point of size which should be fed to ball mills. As to whether coarser material can be fed with satisfactory capacities will depend upon its hardness.
Testing for Concentrating Processes.—The determination of the proper flowsheet for a difficult complex ore is a matter requiring not only careful testing but exercise of the nicest judgment. But refinements in treatment may be carried so far that a flowsheet while technically excellent may not be admirable from a commercial standpoint. That is, the increased saving which has been effected beyond a certain point may have been obtained at an expense which more than eats up the ore receipts from treatment refinements. The ideal ore for concentration is one where the mineral or other substance to be saved is in a pure state and arranged in masses or particles in the matrix or gangue. The larger such masses are the more amenable the ore is to concentrating methods. For gravitational methods the greater the difference in specific gravity between the mineral or material to be saved and the gangue the more readily it will yield to them. In the employment of the other principles employed in concentration which have been mentioned at the beginning of this section gravity is not the sole factor nor even the principal factor in concentration or an index of the ease with which concentration can be effected. Galena, sp. gr. 7.25 to 7.7, "floats"